The production of metals through its electrodeposition in the cathode of an electrolysis cell is an technique with practically a century of industrial history.
The metals are produced via electrolysis of either dissolved or molten salts, depending on their chemical peculiarities. The cations move from the electrolyte toward the cathode surface, where they are reduced into elemental metals, discharged there and removed, continuously or discontinuously, from there.
When molten salt is used as the anolyte, the deposited metal is usually recovered in liquid state, and it is poured molten from the cell. This is the case for aluminum and magnesium electrowinning.
There is an ample range of other metals, however, that are electrowon from liquid solutions, mainly aqueous ones, and discharged as solid metals. The morphology of this solid can be as compact as plates, or any variety of spongy, porous deposits.
The invention that is the subject of this patent deals with the electrowinning of solid metals from solutions, whatever their form. It could be applied to mercury electrowinning as well, but obviously it is an exception.
The design of an industrial electrowinning cell requires solving a number of engineering problems. The main one is the conflict between the conflicting requirements imposed by two aspects of the operation:
The need for minimizing investment costs demands that cathode surface be as wide as possible. On the other hand, the need for minimizing operating costs demands that the anode-cathode distance be as small as possible, in order to avoid useless energy costs derived from the ohmic resistance in that space.
When engineers try to satisfy both demands, the result will be a wide cathodic surface (in the order of 1 m.sup.2 /unit) separated from the corresponding anodic surface, or any separating surface between anode and cathode by merely 20-30 mm gap.
However, this solution poses a strong constraint for the electrolyte access to the whole cathodic surface. The required feed to every spot of the surface is made from some peripheral point, and it is hindered by the small cross-section available for the flow. The electrolyte must be present with constant composition in the vicinity of the whole electrodic surface. When flow restrictions cause local concentration depletion, the electrochemical conditions are changed, and the results may be degraded, ranging from loss of current efficiency to change in the deposit composition.
Techniques to overcome such conflict have been developed over the years, as common practice in electrowinning installations and patented inventions. Among the more common procedures, it is worthwhile to cite the high rate of catholyte recirculation, or nozzle injection in the interelectrodic space, or gas bubbling there; all of them aiming for a greater turbulence degree, in such a way that mass transport is enhanced.
This problem is a typically cathodic one, usually not applicable to the anodes, as gas is usually produced at the anode, and its bubbling produces enough turbulence to overcome this problem. But similar considerations could be raised when anodic product is not a gas.
The problem described above is important even when smooth, regular flat metal deposits are formed on the cathodic surface. But the disadvantage is greater in cases where the metal deposits grow in porous, spongy, or highly dentritic forms. The irregularities of the surface increase progressively the resistance to the electrolyte flow, up to points of damage, due to extensive restriction and large local concentration depletion.